Method and apparatus for plasma-mediated thermo-electrical ablation

ABSTRACT

An apparatus and method for cutting a material including conducting and non-conducting materials such as biological tissue, cellulose or plastic while the material is submerged in a conductive liquid medium. The apparatus has a cutting electrode with an elongate cutting portion having an aspect ratio (length to width) of 1 or more and a return electrode. The two electrodes are immersed in the conductive medium and a voltage is applied between them to heat the medium, thus producing a vapor cavity around the elongate cutting portion and ionizing a gas inside the vapor cavity to produce a plasma. The voltage applied between the electrodes is modulated in pulses having a modulation format selected to minimize the size of the vapor cavity, its rate of formation and heat diffusion into the material while the latter is cut with an edge of the elongate cutting portion. The modulation format includes pulses ranging in duration from 10 μs to 10 ms, as well as minipulses and micropulses, as necessary. The apparatus and method of invention allow the user to perform efficient thermal ablation and electrosurgical procedures in particular at power levels as low as 10 mW with minimal thermal and cavitation damage.

FIELD OF THE INVENTION

[0001] The present invention relates to an apparatus for cuttingmaterials including biological tissue by thermo-electrical ablation withthe aid of a plasma produced around a cutting electrode and to a methodfor driving such electrode with appropriate pulses.

BACKGROUND

[0002] The cutting of materials with the aid of cutting electrodesenergized by a suitable power source is a known technique that is beingsuccessfully employed, e.g., in the field of electrosurgery. Typicalelectrosurgical devices apply a electrical potential difference or avoltage difference between a cutting electrode and a patient's groundedbody (monopolar arrangement) or between a cutting electrode and a returnelectrode (bipolar arrangement) to deliver electrical energy to the areawhere tissue is to be cut. The voltage is applied either as a continuoustrain of high frequency pulses, typically in the RF range, or as directcurrent (DC).

[0003] The prior art provides a number of exemplary designs of bipolarelectrosurgical electrodes. For example, U.S. Pat. No. 5,108,391describes a bipolar treating apparatus with a first active electrode anda second return electrode having exposed distal ends to define a bipolartip for electrosurgically treating tissue. U.S. Pat. No. 5,700,262describes a bipolar electrode with fluid channels for performingneurosurgery. Additional information about bipolar electrosurgicaldevices and knives can be found, e.g., in U.S. Pat. Nos. 4,202,337 and4,228,800 as well as numerous other open literature sources.

[0004] Depending on the conditions, the application of a voltage to amonopolar electrode or between the cutting and return electrodes of abipolar electrode produces a number of physical phenomena. Most priorart devices take advantage of one of these phenomena to perform the cut.In particular, one class of devices uses a gas stream that is generatedaround the cutting electrode. For example, U.S. Pat. No. 5,217,457describes an electrosurgical apparatus using a stream of gas thatshrouds the electrode and an electrosurgical apparatus incorporatingthis electrode for cutting biological tissue. U.S. Pat. No. 5,088,997also teaches the use of a stream of gas for electrosurgical proceduresfor coagulating or cutting biological tissue. On the other hand, U.S.Pat. No. 5,300,068 teaches an electrosurgical apparatus for cuttingtissue and for ablating occlusions using arc discharges produced on amonopolar electrode in response to a train of pulses. Taking advantageof a yet different phenomenon, U.S. Pat. No. 6,352,535 teaches a methodand device for electro microsurgery in a physiological liquidenvironment that uses high voltage electrical discharges ofsub-microsecond duration in a liquid medium to produce cavitationbubbles. The cavitation bubbles have a size in the sub-millimeter rangeand are used for high-speed precision cutting with an inlaid discelectrode.

[0005] In addition to taking advantage of different phenomena to performthe cut, prior art devices employ various techniques for generating andapplying the voltage to the electrode or electrodes. U.S. Pat. No.6,135,998 teaches an electrosurgical device which uses extremely shortmonopolar voltage pulses, 2i typically shorter than 200 ns, to drive anelectrode having an inlaid disc geometry. This invention attempts tomitigate some of the negative cavitation effects, such as the damagingjets formed after the collapse of the cavitation bubble. U.S. Pat. No.5,108,391 describes a high frequency generator for tissue cutting andfor coagulating in high-frequency surgery. This device uses an electricarc discharge to perform the cutting operation. U.S. Pat. No. 6,267,757teaches a device which uses radio-frequency (RF) ablation forrevascularization. It employs a source, which delivers at least oneburst of RF energy over an interval of about 1 to about 500 ms, andpreferably about 30 to about 130 ms. This device has an elongatedinsulated, electrical conducting shaft with an uninsulated distal tip,which is configured to emit the RF energy. U.S. Pat. No. 6,364,877 alsodescribes the use of high frequency pulses applied in a continuousmanner. The teaching found in U.S. Pat. Nos. 5,697,090 and 5,766,153suggests that a continuous train of high frequency pulses can be pulsedat a rate sufficient to allow the electrode to cool.

[0006] Unfortunately, despite all the above teachings, electrosurgicalmethods and apparatus generally suffer from an inability to control thedepth of tissue damage (necrosis) in the tissue being treated. Mostelectrosurgical devices described above rely on a gas jet, an arcdischarge or cavitation bubbles to cut, coagulate or ablate tissue. Suchimprecise cutting methods cause tissue necrosis extending up to 1,700 μminto surrounding tissue in some cases.

[0007] In an effort to overcome at least some of the limitations ofelectrosurgery, laser apparatus have been developed for use inarthroscopic and other procedures. Lasers do not suffer from electricalshorting in conductive environments and certain types of lasers allowfor very controlled cutting with limited depth of necrosis. U.S. Pat.No. 5,785,704 provides an example of a laser used for performingstereotactic laser surgery. Unfortunately, lasers suffer fromlimitations such as slow operating speed, inability to work in liquidenvironments, high cost, inconvenient delivery systems and other defectsthat prevent their more universal application. For these reasons, itwould be desirable to provide improved apparatus and efficient methodsfor driving an electrosurgical apparatus for ablating tissue in a highlycontrolled and efficient manner while minimizing tissue damage.

[0008] The prior art has attempted to provide for more controlledelectrosurgery by relying on plasma-mediated cutting and ablation ofsoft biological tissue in conductive liquid media at low temperatures.The fundamentals of this approach, which is used predominantly in thecontinuous pulse regime, and various embodiments employing it aredescribed in the patents of Arthrocare including U.S. Pat. Nos.5,683,366; 5,697,281; 5,843,019; 5,873,855; 6,032,674; 6,102,046;6,149,620; 6,228,082; 6,254,600 and 6,355,032. The mechanism of lowtemperature ablation is called “coblation” and is described as electricfield-induced molecular breakdown of target tissue through moleculardissociation. In other words, the tissue structure is volumetricallyremoved through molecular disintegration of complex organic moleculesinto non-viable atoms and molecules, such as hydrogen, oxides of carbon,hydrocarbons and nitrogen compounds. This molecular disintegrationcompletely removes the tissue structure, as opposed to transforming thetissue material from solid form directly to a gas form, as is typicallythe case with ablation (see U.S. Pat. No. 5,683,366). More specifically,this mechanism of ablation is described as being associated with twofactors: (1) “photoablation” by UV light at 306-315 nm and visible lightat 588-590 nm produced by the plasma discharge; and (2) energeticelectrons (e.g. 4 to 5 eV) can subsequently bombard a molecule and breakits bonds, dissociating a molecule into free radicals, which thencombine into final gaseous or liquid species (see U.S. Pat. No.5,683,366). Surface temperature of tissue in this process is maintainedbetween 40-70° C. This type of ablation mechanism has low rate of tissuedissection and a very limited applicability to hard tissues such as, forexample, bones.

[0009] Despite these new advances the electrosurgical techniques arestill experiencing a number of problems remain. First and foremost, theamount of power required to operate the prior art cutting devicesremains in a high range of several Watts which precludes applications ofthese devices to such delicate organs as an eye. Second, the devicesexhibit large energy and heat losses. These high losses translated intoexcessive power deposition into the tissue being ablated. Additionalheat losses to the hand piece are also substantial. Third, even the bestprior art devices operating at the lowest power levels have difficultiescutting hard biomaterials like bones and non-conducting materials suchas cellulose or plastics.

[0010] Increasingly sophisticated surgical procedures create a growingdemand for more precise and less traumatic surgical devices. Thecritical importance and delicate nature of the eye makes the demand forprecision and safety of intraocular microsurgical instrumentationparticularly important. For these and other reasons, it would be a majoradvance in the art to provide an apparatus and method for ablatingmaterials at low power levels. It would be particularly useful toprovide such apparatus and method that reduces heat losses to thematerial being cut as well as into the surroundings and, especially thehand piece.

[0011] Furthermore, it would also be an advance to expand the range ofmaterials that can be ablated to include biological tissue, celluloseand plastics.

OBJECTS AND ADVANTAGES

[0012] In view of the above shortcomings of the prior art, it is anobject of the invention to produce a cutting apparatus and provide amethod for operating it to achieve efficient thermal ablation at lowpower levels, e.g., ranging down to 10 mW, by overheating andevaporation in various types of materials including biological tissue.Specifically, it is an aim of the invention to minimize the damage zoneproduced during the cutting process by using plasma-assisted cutting andminimizing heat losses into the material being cut as well as thesurroundings and the hand piece.

[0013] It is another object of the invention to provide a modulationformat for pulsed operation of the cutting apparatus to minimize adverseeffects in cutting biological tissue.

[0014] It is yet another object of the invention to reduce the voltagenecessary for ionization of the gas to derive the plasma.

[0015] Yet another object of the invention is to provide a versatilecutting electrode geometry for efficient cutting and removal ofmaterial. Some of these electrodes and the driving waveforms arespecifically designed for applications in eye surgery.

[0016] These and other objects and advantages will become apparent uponreview of the following description and figures.

SUMMARY OF THE INVENTION

[0017] The objects and advantages of the invention are achieved by amethod for cutting a material including conducting and non-conductingmaterials such as biological tissue, cellulose or plastic. Duringcutting the material is submerged in a conductive liquid medium. Themethod involves providing a cutting electrode with an elongate cuttingportion and a return electrode. The elongate cutting portion has anaspect ratio of length to width larger than 1 and preferably larger than5. Thin cutting electrode allows for dissection of tissue with lowenergy deposition. The two electrodes are immersed in the conductivemedium and a voltage is applied between them such that the conductiveliquid medium is heated to produce a vapor cavity around the elongatecutting portion and to ionize a gas inside the vapor cavity to produce aplasma. The presence of the plasma maintains electrical conductivitybetween the electrodes. The voltage applied between the electrodes ismodulated in pulses having a modulation format selected to minimize thesize of the vapor cavity, the rate of formation of the vapor cavity andheat diffusion into the material as the material is cut with an edge ofthe elongate cutting portion of the cutting electrode.

[0018] The modulation format includes pulses having a pulse duration inthe range from 10 μs to 10 ms. Preferably, the pulses are composed ofminipulses having a minipulse duration in the range between 0.1 and 10μs and an interval ranging from 0.1 to 10 μs between the minipulses.Preferably, the minipulse duration is selected in the rangesubstantially between 0.2 and 5 μs and the interval between them isshorter than a lifetime of the vapor cavity. The peak power of theminipulses can be varied from minipulse to minipulse.

[0019] When the method is used for cutting biological tissue it ispreferable to use minipulses with alternating polarity. In other words,the modulation format contains minipulses that exhibit alternatingpositive and negative polarities. This modulation format limits theamount of charge transfer to the tissue and avoids various adversetissue reactions such as muscule contractions and electroporation. Infact, additional devices for preventing charge transfer to thebiological tissue can be employed in combination with this modulationformat or separately when the method of invention is applied inperforming electrosurgery.

[0020] In the same or in an alternative method of the invention theminipulses are further made up of micropulses. When the modulationformat includes micropulses it is preferred that they have a durationranging between 0.1 and 1 μs.

[0021] It is well-known that spark discharges develop in advance of anarc discharge. In accordance with the invention it is preferable toadjust the modulation format to permit spark discharges while preventingarc discharges. For example, the modulation format such as minipulseduration and peak power are adjusted to permit spark discharges whileavoiding arc discharges. Furthermore, the voltage and the modulationformat are selected such that the temperature of the elongate cuttingportion of the cutting electrode and of the plasma are maintainedsignificantly above the boiling temperature of water. Preferably, thetemperature of the elongate cutting portion is maintained between about100 and 1,000° C.

[0022] The invention further provides an apparatus for cutting materialssubmerged in the conductive liquid medium. The apparatus is equippedwith the cutting electrode with the elongate cutting portion and returnelectrode. A voltage source is used for applying the voltage between thecutting and return electrodes to produce the vapor cavity with plasma. Apulse control is provided for controlling the modulation format of thevoltage applied between the electrodes. The pulse control has a peakpower control and a duration control for adjusting pulse power, pulseduration and pulse interval.

[0023] The shape of the cutting electrode and the elongate cuttingportion can vary according to the material being cut. For a number ofelectrosurgical applications the elongate cutting portion should have awidth between 1 μm and 200 μm and preferably between 10 μm and 100 μm.The elongate cutting portion can have various cross sections includingcircular, e.g., it is in the form of a wire. In these cases entirecutting electrode can be in the form of a wire electrode. Application ofa thin wire as a cutting electrode allows for reduction of powerrequired for tissue dissection and reduces the depth of the damage zoneproduced at the edges of the cut. In order to perform certain types ofcuts the elongate cutting portion can have one or more bends. Forexample, in certain electrosurgical applications the elongate cuttingportion can be L-shaped or U-shaped. In some embodiments the elongatecutting portion can form a loop, e.g., it can be a looped wireelectrode. In some embodiments it is advantageous to provide a devicefor advancing the wire electrode such that a length of the wire used forcutting can be adjusted during the application, when required. Suchadjustment affects the impedance of the electrode and can be used forcontrol of power dissipation. In addition, a fresh portion of the wirecan be extended to replace the eroded portion. In one particularembodiment, the elongate cutting portion and the terminal portion ofreturn electrode are both shaped into a shape suitable for capsulotomy.

[0024] In embodiments where transferring charge to the material shouldbe avoided, e.g., when the material being cut is biological tissue, theapparatus has a device for preventing charge transfer to thenon-conducting material. For example, a circuit with a separatingcapacitor, e.g., an RC-circuit, can be used for this purpose. Thedetails of the invention are discussed below with reference to theattached drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 shows a three-dimensional view of an apparatus according tothe invention employed in cutting biological tissue.

[0026]FIG. 2 is a graph illustrating a pulse modulation format accordingto the invention.

[0027]FIG. 3 is a graph indicating the qualitative dependance of thecavitation bubble diameter and heat diffusion on duration of the pulse.

[0028]FIG. 4A is a graph illustrating the conversion of electricalenergy of the discharge (1 mJ) into the mechanical energy of the bubblemeasured as a function of pulse duration for the apparatus of FIG. 1.

[0029]FIG. 4B is a graph illustrating the cavitation bubble size, energydeposition and heat diffusion as a function of pulse duration for theapparatus of FIG. 1.

[0030]FIG. 5 is a photograph of the use of cutting electrode withelongate cutting portion for cutting paper.

[0031]FIG. 6 is a graph of a pre-pulse and post-pulse used in accordancewith the invention.

[0032]FIG. 7 is a graph illustrating the use of micropulses inaccordance with the invention.

[0033]FIG. 8 is a graph illustrating the use of minipulses ofalternating polarity in accordanc with the invention.

[0034]FIG. 9 illustrates an apparatus of the invention used in cutting amaterial.

[0035]FIG. 10 illustrates an apparatus of the invention using a shapedcutting electrode.

[0036] FIGS. 11A-C are partial views of alternative embodiments inaccordance with the invention.

[0037]FIG. 12 illustrates an apparatus of the invention designed forcapsulotomy.

DETAILED DESCRIPTION

[0038]FIG. 1 illustrates an apparatus 10 for cutting a material 12submerged in a conducting liquid medium 14. In this embodiment material12 is a biological tissue made up of various types of tissue includingmuscle tissue 12A, nerve tissue 12B, bone 12C and soft tissue 12D. Ingeneral, however, material 12 can be any conducting or non-conductingmaterial which requires cutting and can include materials such ascellulose, e.g., wood and cellulose-based materials as well as varioustypes of non-conducting plastics. Liquid medium 14 can be any type ofelectrolyte. In the present embodiment, liquid medium 14 is aphysiological medium, for example an isotonic saline solution.

[0039] Apparatus 10 has a cutting electrode 16 with an elongate cuttingportion 18. In the present embodiment, entire cutting electrode 16 is inthe form of a wire electrode with circular cross section defined by aradius re. The material of wire electrode 16 can be any suitableconductor such as a metal like Tungsten, Titanium, Molybdenum, etc. oran alloy. In the present embodiment electrode 16 is made of Tungstenwire. Cutting electrode 16 is surrounded by an insulating layer 20 and areturn electrode 22. Insulating layer 20 can be any dielectric materialor combination of materials such as ceramic, plastic, glass, and/or airthat provide electrical insulation between electrodes 16 and 22.Electrodes 16 and 22 are arranged coaxially along a center line 26.Cutting portion 18 protrudes beyond insulating layer 20 and returnelectrode 22. In fact, a length L of elongate cutting portion 18 isexposed. The aspect ratio of length L to width w (w=2r_(e)) of cuttingportion 18 is at least 1 and preferably more than 5.

[0040] A voltage control unit 24 is connected to cutting electrode 16and to return electrode 22. Voltage control unit 24 has a voltagegenerator for producing a voltage to be applied between electrodes 16,22. Unit 24 also has a pulse control for pulsing the voltage inaccordance with a predetermined modulation format, as described below.The pulse control has a peak power control and a duration control foradjusting a pulse power, a pulse duration τ and a pulse interval.

[0041] The operating principle of the method of invention is based uponformation of a thin layer of a plasma 28 around elongate cutting portion18. To achieve this goal, electrodes 16, 22 of apparatus 10 are immersedin conductive medium 14 where tissue 12 is submerged and a voltage isapplied between electrodes 16, 22 such that medium 14 is heated toproduce a vapor cavity 30 around cutting portion 18. During heating anamount of medium 14 is vaporized to produce a gas 32 inside vapor cavity30. In the present case medium 14 is saline and thus gas 32 is composedpredominantly of water vapor, a small amount of oxygen and hydrogen andtrace amounts of NaCl. The layer of gas 32 is ionized in the strongelectric field around cutting electrode 16 to make up the thin layer ofplasma 28. Because plasma 28 is electrically conductive it maintainselectrical conductivity between electrodes 16, 22.

[0042] In contrast to the prior art, it is important that the size andrate of formation of vapor cavity 30 as well as heat diffusion intotissue 12 be minimized. The size and rate of formation of cavity 30 arerelated and can be minimized by modulating the voltage applied betweenelectrodes 16, 22 by the pulse control of unit 24 in accordance with amodulation format. Specifically, pulse control modulates the appliedvoltage in pulses 34, as shown in FIG. 2. The modulation format ofpulses 34 is selected to minimize the size of vapor cavity 30, the rateof formation of vapor cavity 30 and also heat diffusion into tissue 12.

[0043] To better understand the principles behind selecting themodulation format to achieve this minimization we now refer to thequalitative graphs in FIG. 3. Graph 38 illustrates the radius aroundelongate cutting portion 18 to which heat diffuses as a function ofduration τ of pulse 34. As duration τ of pulse 34 increases heatdiffuses deeper into tissue 12. This diffusion of heat causes thermaldamage to tissue 12 and it is to be avoided. It should be noted, thatthe application of a long train of very high frequency pulses, e.g., RFpulses, will effectively act as one long pulse whose duration is equalto the entire duration of the pulse train. Hence, prior art devicesoperating in the continuous regime and applying RF pulses (seeBackground section) suffer from high heat diffusion and consequentlycause large thermal damage to surrounding tissue.

[0044] Graph 40 illustrates the maximal radius of vapor cavity in thiscase also referred to as bubble 30 (see FIG. 1), or cavitation bubble,which is formed at constant pulse energy around cutting electrode 16.Now, the radius of cavitation bubble 30 initially increases withincreasing pulse duration τ and then decreases and approaches zero asduration τ of pulse 34 tends to infinity (continuous current). Graphs 38and 40 intersect at a pulse duration τ_(c) at which heat diffusion isstill relatively insignificant while the radius of bubble 30 is alreadysmall enough not to cause significant tissue damage. Thus, by chosingduration τ of pulse 34 in a range 42 around τ_(c) heat damage andmechanical damage due to cavitation bubble 30 are minimized. In fact,choosing duration τ of pulses 34 so as not to produce large cavitationbubble 30 is equivalent to minimizing the size and rate of formation ofvapor cavity 30. A person skilled in the art will appreciate that theexact shape of graphs 38, 40 and range 42 will vary depending onspecific parameters such the exact composition of tissue 12, salinity ofelectrolyte 14 and geometry of electrode 16.

[0045]FIG. 4A shows a graph 44 of the conversion of the electricalenergy of the discharge for a discharge energy equal to 1 mJ andelectrode 16 diameter of 25 μm into mechanical energy of bubble 30measured as a function of duration τ of pulse 34. Efficiency of theconversion decreases with increasing duration τ of pulse 34 once pulse34 is longer than about 3 μs. In FIG. 4B the radius of single bubble 30is illustrated by graph 38′ as a function of pulse duration τ in a 1 mJdischarge. At pulse duration τ above 50 μs a sequence of bubbles isformed with maximal radii reducing with increasing duration τ, asdepicted by separate rhombuses. Graph 48 represents the penetrationdepth into material 12 of electric field E(r), here equal to radiusr_(e) of cutting portion 18. Graph 40′ represents the radius aroundcutting portion 18 to which heat diffuses assuming constant temperatureof cutting portion 18 in one dimensional geometry.

[0046] A range 42′ in which pulse duration τ is optimized and in whichboth cavitation and heat diffusion are comparable with field penetrationdepth is between 50 μs and 2 ms. Under different conditions range 42′will vary, but optimal duration τ of pulses 34 will generally fallbetween 10 μs and 10 ms. In this range 42′ the size and rate offormation of vapor cavity 30 as well as heat diffusion into tissue 12are minimized. The thermal damage zone in tissue 12 due to heatdiffusion is dependent mostly on duration τ of pulse 34. Specifically,varying duration τ of pulses 34 between 0.1 and 100 ms changes the depthof the heated zone in tissue 12 between 10 and 300 μm ranging fromsingle cellular layer with no hemostatic effect to complete hemostasisin most of tissues 12A, 12B, 12C and 12D.

[0047] Referring back to FIG. 2, pulses 34 can be delivered in variousmodulation formats including continuous pulses or bursts of short pulsesor minipulses 50. Preferably, pulses 34 are separated by a separation 49of at least 1 ms, and preferably at least 10 ms while pulses 34themselves are composed of a number of minipulses 50, as shown. Theamplitude and duration of minipulses 50 determine the spatial extent anddensity of plasma 28. To avoid excessive overheating of tissue 12 themodulation format is adjusted so that plasma 28 is maintained at theregime of streamer and spark discharges but the arc discharges areprevented. Specifically, duration and peak power of minipulses 50 areadjusted to permit spark discharges and to prevent arc discharges. Inmost cases, limiting the duration of minipulses 50 to less than severalμs will accomplish this goal. In fact, the duration of minipulses 50should be kept in the range between 0.1 and 10 μs and preferably between0.2 and 5 μs. The interval between minipulses 50 is preferably selectedin the range between 0.1 and 10 μs. Such short times are sufficient forionization and development of the spark discharges but not for creationof the arc discharge.

[0048] An arc discharge is a highly luminous and intensely hot dischargeof electricity between two electrodes, in this case between electrode16, and more precisely its cutting portion 18, and return electrode 22.The arc discharge is initiated when a strong electric forces drawelectrons from one electrode to the other, initiating the arc. It istypically a continuous discharge characterized by high current and lowvoltage across the arc. On the other hand, a spark discharge has a highvoltage and short duration.

[0049] If the intervals between minipulses 50 do not exceed a lifetimeof vapor cavity 30 the ionization will be maintained by minipulses 50until vapor cavity 30 collapses. Hence, in any situation, the intervalsbetween micropulses 50 should be kept shorter than the lifetime of vaporcavity 30. For example, the lifetime of a 100 μm wide vapor cavity 30 isabout 10 μs, thus minipulses 50 should be delivered at intervals notlonger than 10 μs when working with such cavity width.

[0050] In contrast to prior art devices, apparatus 10 cuts tissue 12using a side or edge of cutting portion 18, i.e., the entire length L ofcutting portion 18 is available for performing the cut. Rapid andefficient ablation of tissue 12 is achieved when the temperature ofcutting portion 18 and layer of plasma 28 around it are maintainedsignificantly above the boiling temperature of water. In order to ensurethat such temperature is efficiently maintained cutting portion 18 islong and thin, i.e., has a small radius—a few tens of microns—and anaspect ratio (length to width) of at least 1 and preferably at least 5.Such thin cutting portion 18 also reduces the amount of heat flowthrough the metal back into a hand piece (not shown).

[0051] In fact, heat flow W through cutting portion 18 is equal to:

W=χΔTS/L,

[0052] where S=πd²/4 is the cross section area of cutting portion 18. Inthe above equation χ is the coefficient of thermal conductivity and ΔTis the difference in temperature between the hot and cold parts of wireelectrode 16, L is the length of cutting portion 18 and d=2r_(e).Evaporation rate of tissue 12 is equal to:

V=Ldv,

[0053] where v is the velocity of advance of cutting portion 18 throughtissue 12. The amount of power deposited in tissue 12 to achieve suchevaporation rate is:

P=V·ρ(CΔT ₁+δ),

[0054] where ρ is the density of tissue 12, C is its heat capacity, ΔT₁is the temperature rise from ambient to 100° C., and δ is the specificheat of evaporation (for tissue mostly composed of water the specificheat of evaporation of water δ=2.26×10³ J/g can be used in thecalculation). To prevent cooling of cutting portion 18 and of layer ofplasma 28 by heat transfer via electrode 16, power deposition P shouldbe kept significantly larger than the heat flow W, i.e., P>>W. In thepresent embodiment electrode 16 is made of Tungsten which has a heatconductivity χ=178 W/m×K, ΔT₁=70° K and cutting portion 18 is advancedthrough tissue 12. For example, at ΔT=250° K and v=1 mm/s one obtainsthe condition L²/d>>14 mm from the above equations. Therefore, toefficiently prevent cooling when cutting portion 18 has a length L=1 mmthe diameter d=2r_(e) of cutting portion 18 should be less than 70 μm.For ΔT=70° K and the rest of the parameters remaining the same we willobtain the conditions L²/d>>4 mm. This means that a 1 mm long cuttingportion 18 should not be thicker than 250 microns.

[0055] The temperature of cutting portion 18 can be maintained as low asabout 100° C., but it is preferably much higher, ranging up to 1000° C.In this temperature range tissue 12 is rapidly evaporated and thusablated. Due to the turbulent flow of liquid boiling at the edges ofvapor cavity 30 the interface with tissue 12 is only minimallyoverheated and damaged.

[0056] In the regime of heating produced by plasma 28 the temperature ofcutting portion 18 is stabilized by a naturally occurring negativefeedback mechanism as follows. In the areas where the vapor sheet ofcavity 30 becomes thinner, the electric impedance is reduced and thusmore current flows. The increased current results in increasedgeneration of Joule heat in that area, and thus more electrolyte 14 isevaporated thereby increasing the thickness of vapor cavity 30 in thatarea. This mechanism stabilizes the thickness of vapor cavity 30 aroundcutting portion 18 and the thermal conditions of cutting portion 18.When tissue 12 is brought into ionized vapor cavity 30, thus reducingits thickness in that area, more current flows into tissue 12 than intoplasma 28, since the impedance of tissue (which is typically similar tothat of electrolyte 14) is much lower than that of plasma 28. Thus, moreheat is generated in the area where tissue 12 is positioned inside vaporcavity 30.

[0057] Application of thin elongated electrode (for example a wireelectrode) allows for minimization of the amount of material evaporatedduring tissue dissection as well as for minimization of the depth of thedamage zone produced at the edges of the cut, as shown below. In thepresent embodiment, the electric field E(r) around cylindrical cuttingportion 18 is reciprocal to the distance from it, and the density ofJoule heat generated in liquid by the discharge is reciprocal to thesquare of that distance. Thus, thinner cutting portion 18 results in amore confined energy deposition. In fact, the electric field E(r) aroundcylindrical cutting portion 18 scales with distance r as follows:${E = \frac{E_{e}r_{e}}{r}},$

[0058] where E_(e) is the value of the electric field on the surface ofcutting potion 18. Thus, the difference in voltage on the surface ofcutting portion 18 and at a distance R from electrode 16 is:U_(e) − U_(R) = ∫_(R)^(r)E(r)  r = E_(e)r_(e)(ln   R − ln   r_(e)).

[0059] The electric field becomes spherical at distances larger thanlength L of cutting portion 18, and thus it can be assumed that theelectric potential drops to zero for distances larger than L. Therefore,the electric field E_(e) at the surface of cutting portion 18 is:$E_{e} = {\frac{U_{e}}{r_{e}( {{\ln \quad L} - {\ln \quad r_{e}}} )}.}$

[0060] The power density w of the Joule heat generated in electrolyte 14is then:${w = {{j^{2}\gamma} = {\frac{E_{e}^{2}}{\gamma} = \frac{U_{e}^{2}}{{r_{e}^{2}( {{\ln \quad L} - {\ln \quad r_{e}}} )}^{2}\gamma}}}},$

[0061] where j is the current density and γ is the resistivity ofelectrolyte 14. The minimal energy density for overheating of thesurface layer of electrolyte 14 (assumed to be water) by pulse 34 ofduration τ is:

A=w·τ=ρ·C·ΔT,

[0062] where ΔT is the total temperature rise in the surface layer ofelectrolyte 14 during pulse 34, ρ is the density of water and C is itsheat capacity. Therefore, the voltage U required for initiation ofvaporization during pulse 34 of duration τ is:

U=r _(e)(1nL−lnr _(e)){square root}{square root over (ρ·C·ΔT·γ/τ)}.

[0063] The voltage U and associated energy deposition can be reduced bydecreasing the radius r_(e) of cutting portion 18. In general, ambienttemperature is about 30° C. when operating in biological tissue 12 of alive subject, boiling temperature is 100° C., ρ=1 g/cm³, C=4.2 J/(g·K)and γ≈70 Ohm·cm. With these values we obtain A≈300 J/cm³ and U=260 V forpulse 34 of duration τ=0.1 ms, r_(e)=25 μm and L=1 mm.

[0064] Since the electric field is reciprocal to the distance from thecylindrical electrode, the field efficiently penetrates into theelectrolyte to the depth similar to the radius of the electrode. Thisminimal amount of energy required for creation of the vapor cavityaround the electrode is:

A=w·τ=ρ·C·ΔT·π·d ² ·L,

[0065] where d is the diameter of the electrode. Minimal depth of thedamage zone at the edges of the cut will thus be similar to the radiusof the electrode. Thus, reduction in radius of the electrode results inreduction in the power consumption and in the width of the damage zoneproduced at the edges of the cut. The threshold voltage U_(th) requiredfor reaching the threshold electric field E_(th) to ionize gas 32 andproduce plasma 28 is:

U _(th) =E _(th) r _(e)1n(R/r _(e)),

[0066] where R is the radius of vapor cavity 30, as shown in FIG. 1.Threshold voltage U_(th) can be decreased by reducing radius r_(e) ofcutting portion 18. This also results in a lower power dissipation andconsequently in a smaller damage zone in tissue 12.

[0067] Vapor cavity 30 filled with plasma 28 and surrounding cuttingportion 18 of cutting electrode 16 serves three major functions. First,it thermally isolates cutting electrode 16 from electrolyte 14 thusallowing for efficient heating. Second, the electric impedance of plasma28 is much higher than that of tissue 12, thus Joule heating isgenerated mostly in plasma 28 and not in the surrounding liquidenvironment. Third, since both electrical and thermal conductivity oftissue 12 is much higher than that of a vapor (gas 32), when tissue 12is introduced inside vapor cavity 30 with plasma 28 it attracts bothelectric current and heat flow, which results in fast overheating andevaporation.

[0068] Another advantage of the cylindrical geometry of cuttingelectrode 16 as compared to prior art point sources (inlaid discgeometry) is that it allows for cutting tissue 12 with the side edge ofcutting portion 18. Prior art point sources (see U.S. Pat. No.6,135,998) produce a series of perforations when a train of pulses isapplied. These perforations do not always form a continuous cut leavingbehind bridges between the edges of the cut. To dissect these bridgesthe secondary scans are required and targeting these thin and oftentransparent straps of tissue is very difficult and time consuming.Cylindrical cutting portion 18 solves this problem by enabling thecutting by its edge and not only by its end or tip.

[0069] In order to reduce unnecessary energy deposition, e.g., duringelectrosurgery, the voltage of source 24 can be set to a level which issufficient for ionization of only a thin layer of vapor. Thus, in areaswhere vapor cavity 30 is too large (typically above several tens ofmicrons) no plasma 28 will be formed. As a result, ionization andformation of plasma 28 will only take place in the areas of proximity orcontact between generally conductive tissue 12 and conductive cuttingportion 18. In other parts of vapor cavity 30 gas 32 will not be ionizedand thus it will electrically insulate cutting electrode 18 preventingheat deposition into the surrounding environment. FIG. 5 illustratescutting electrode 16 with cutting portion 18 of radius r_(e)=25 μmimmersed in isotonic saline solution touching the edge of a material 52.In this case material 52 is made of cellulose and is in fact a sheet ofpaper. Cutting portion 18 is touching an edge of paper 52 that is about250 μm thick. As is clearly seen, plasma 28 is generated only in thearea of contact between cutting electrode 18 and paper 52.

[0070] To further reduce the energy deposition cavity 30 can be createdby electrochemical generation of gas 32, i.e., by electrolysis of water,rather than by its vaporization. For this purpose the pulse control andsource 24 can vary the voltage between parts of the pulse or evenbetween two successive pulses, as shown in FIG. 6. First, source 24applies a pre-pulse 54 of relatively low voltage. This low voltageshould be sufficient for electrolysis and can be in the range of severaltens of Volts. In accordance with well-known principles, the applicationof such low voltage will yield oxygen gas on the anode and hydrogen gason the cathode. The user can choose whether to use oxygen or hydrogen asgas 32 by selecting the polarity of pre-pulse 54, such that cuttingportion 18 is either the anode or cathode. It should be noted, thatapplying a pulse composed of minipulses with alternating polarity (seeFIG. 8 and below description) will generate a mixture of oxygen andhydrogen.

[0071] Next, pulse control and source 24 increase the voltage to arelatively high level in a post-pulse 56. The voltage of post pulse 56can be in the range of several hundred Volts to complete the formationof vapor cavity 30 and to ionize gas 32 to form plasma 28. A sequence ofcombination pulses containing pre-pulse 54 and post-pulse 56 can be usedto drive apparatus 10. Alternatively, a single combination pulse can befollowed by a series of regular pulses 34 composed of minipulses 50, asdescribed above. Embodiments of the method taking advantage ofelectrochemical generation of gas 32 around cutting portion 18 ofelectrode 16 obtain a substantial pulse energy reduction.

[0072] The rate of evaporation of electrolyte 14 depends on itstemperature. There is always a delay between the moment when electrolyte14 reaches boiling temperature (boiling temperature of water) and themoment when formation of vapor cavity 30 disconnects the current flowingthrough electrolyte 14 between electrodes 16, 22. When vapor cavity 30forms, gas 32 stops the current flow and prevents further heating. Justbefore this time an additional energy is deposited that leads tooverheating of electrolyte 14 and thus to explosive (accelerated)vaporization. This effect results in formation of a larger vapor cavity30 and turbulence around cutting portion 18 of electrode 16. To preventsuch overheating the energy for initial boiling should be delivered at alower voltage, but as soon as vapor cavity 30 is formed, the voltageshould be increased to achieve fast ionization of gas 32. Severalsolutions can be employed to address this problem.

[0073] In accordance with a first solution, a low impedance line 58, asindicated in dashed line in FIG. 1, is used instead of a standardelectrical connection between the output of pulse generator in unit 24and cutting electrode 16. In accordance to well-known principles, lowimpedance line 58 will cause the rising edge of a pulse to be reflectedfrom the output end if the output impedance is high. This conditionoccurs when vapor cavity 30 is formed and not while electrode 16 is indirect contact with electrolyte 14. The reflection will oscillate withinline 58 with a period determined by its length, and will form a highfrequency (several MHz) modulation.

[0074]FIG. 7 illustrates the effect of line 58 on minipulses 50 in apulse 34. The first set of minipulses 50 does not experience any changesbecause at this time the output impedance is still low (vapor cavity 30not yet formed). Once vapor cavity 30 is formed reflection occurs andmicropulses 60 are generated. As a result, each minipulse 50 gives riseto a series of micropulses 60. The length of line 58 is selected suchthat micropulses 60 have a duration τ_(μ) in the range between 0.1 and 1μs. The voltage of micropulses 60 is twice as high as that of minipulse50. This doubling in voltage of micropulses 60 is beneficial because itaids in ionizing gas 32 to form plasma 28 more rapidly and depositingmore energy in plasma 28 than it was possible with minipulse 50 at thelower constant voltage level. That is because energy depositionincreases as the square of the voltage and only linearly with the amountof time the voltage is applied. Hence, although micropulses 60 areapplied at electrode 16 only about half the time of a minipulse 50,their doubled voltage raises the energy deposition by a factor of four.

[0075] In accordance with another solution an increase in the rate ofionization of gas 32 is achieved by adding a ballast resistor 62 inseries with the load, as shown in dashed lines in FIG. 1.

[0076] The resistance of resistor 62 (R_(ballast)) is selected to behigher than the impedance of the discharge in electrolyte 14(R_(electrolyte)) but lower than in the ionized vapor or gas 32. As aresult, the heating of electrolyte 14 before evaporation will proceed ata lower voltage U_(low):

U _(low) =U(1+R _(ballast) /R _(electrolyte)).

[0077] The reduced voltage will slow the boiling and cause formation ofthinner vapor cavity 30. After evaporation the impedance will greatlyincrease, resulting in an increase of the discharge voltage to a highvalue U_(high):

U _(low) =U(1+R _(ballast) /R _(vapor)).

[0078] At this high voltage ionization of gas 32 will proceed rapidly.Specifically, when cutting portion 18 has a diameter of 50 μm and itslength L=1 mm the impedance of the discharge in saline 14 is about 500Ω, while in plasma 28 it is about 6 kΩ. Thus, for example, a ballastresistor of 1 kΩ will provide output voltages of U_(low)=U/3 andU_(high)=U/1.17, respectively. The lower limit to the voltage appliedduring the heating phase is set by how much the duration of minipulses50 and pulses 34 can be increased without unacceptable thermal damage totissue 12 is caused by increased heat diffusion.

[0079] In yet another embodiment the method of invention is adaptedspecifically for cutting biological tissue 12 containing muscle tissue12A and nerve tissue 12B. It is known that electric excitation of nervetissue 12B leads to contractions in muscle tissue 12A. In order to avoidcontraction of muscle tissue 12A and reduce the risk of electroporationof adjacent tissue the method of invention calls for limiting andpreferably stopping any charge transfer to tissue 12. This is achievedby using minipulses 50 of alternating positive and negative polarities,as illustrated in FIG. 8. Low impedance line 58 can also be used togenerate micropulses 60 when vapor cavity 30 is formed.

[0080] The polarities are set by the voltage source of unit 24 inaccordance with well-known electronics techniques. In the presentembodiment the alternating polarities can be produced by a separatingcapacitor (not shown). The discharge time constant of the RC circuit,where R is the resistance of the discharge, should not exceed theexcitation time of nerve cells in nerve tissue 12B at the appliedvoltage level. A person skilled in the art will appreciate that exact RCtime constant will have to be adjusted on a case-by-case basis. Ingeneral, however, contractions of muscle tissue 12A will be prevented ata voltage level of 500 Volts if the discharge time does not exceed 1 μs.When cutting portion 18 has a diameter of 50 μm and length L=1 mm theelectrical impedance is about 500 Ω, and hence the capacitance ofcapacitor should not exceed 2 nF. It should be noted that in addition topreventing muscular contractions, alternating polarity of minipulses 50reduces the effect of electroporation, as compared to direct current(DC) (only positive or only negative voltage) pulses.

[0081] Various alternatives can be introduced to the apparatus ofinvention depending on the material being cut and the type of cutrequired. For example, in FIG. 9 a cutting electrode 80 of an apparatusanalogous to apparatus 10 is used for performing a circular incision 84in a material 82. The return electrode and liquid conducting medium arenot shown in this drawing. Material 82 is a thin sheet of plastic orbiological material. When used for performing biopsy, a cylindricalbiopsy can be easily obtained in this manner without bleeding.

[0082]FIG. 10 illustrates a cutting electrode 90 having two bends 92, 94to form a U-shaped electrode. The return electrode and liquid conductingmedium are not shown in this drawing. Cutting electrode 90 is used forremoving a large amount of a material 96 with a single cut. U-shapedcutting electrode 90 can be used to minimize the damage to tissue inelectrosurgery and to maximize the lifetime of cutting electrode 90. Inan alternative version a cutting electrode with a single bend can beused to make and L-shaped cutting electrode. In general, bends atvarious angles can be introduced to cutting electrode to perform anydesired type of cut, to approach tissue at various angles and tomanipulate the tissue before and during the cutting.

[0083]FIG. 11A illustrates a portion of yet another apparatus 100 havinga mechanism 102 for advancing a cutting electrode 104. In thisembodiment cutting electrode 104 is a wire electrode. Return electrode106 is in the form of two capillaries through which wire electrode 104is threaded. Capillaries 106 can be used for delivering an electrolyteand/or aspiring fluids during electrosurgery, i.e., capillaries 106 canbe used for irrigation and suction. Cutting electrode 104 forms a loop108 for cutting tissue in accordance with the method of the invention.Mechanism 102 allows the user to refresh cutting electrode as neededduring operation. Exposure time of wire electrode 104 outsidecapillaries 106 should be smaller than its erosion lifetime. It shouldbe noted that mechanism 102 can be used in other embodiments for bothadvancing and retracting the cutting electrode as necessary to maximizeits lifetime and/or retract an eroded electrode.

[0084]FIG. 11B illustrates a portion of an apparatus 110 using a wireelectrode 112 threaded through capillaries 114. Capillaries 114 servethe dual function of return electrode and channels for delivering andaspiring fluids during operation. Apparatus 110 can be used as a framesaw, as required in electrosurgical applications. FIG. 11C illustrates aportion of still another apparatus 120 functioning as a stationaryscissors for both lifting and cutting of tissue. Apparatus 120 has acutting electrode 122 in the form of a wire threaded through twocapillaries 124 functioning as the return electrode. Mechanism 102allows the user to refresh cutting electrode as needed during operation.Exposure time of wire electrode 112 outside capillaries 114 should besmaller than its erosion lifetime. A projection 126 is used for liftingof tissue. Both apparatus 110 and apparatus 120 are operated inaccordance with the method of the invention.

[0085]FIG. 12 illustrates a portion of an apparatus 130 specificallydesigned for capsulotomy. An electrosurgical probe 132 for capsulotomyhas a shape similar to the mechanical tools used for capsulotomy inorder to make its application easy and convenient for surgeons who areused to such mechanical tools (comparison is shown in the topphotograph). Probe 132 has an insulator 134 with external diametervarying between 0.1 and 1 mm, which has a bent tip 136 at the end. Acutting electrode 138 with a diameter varying between 10 to 200 micronsprotrudes from insulator 134 by a distance varying between 20 microns to1 mm. A return electrode 140 can be either a concentric needle or anexternal electrode attached to the eye or somewhere else to the body ofthe patient. Apparatus 130 protects the tissue located above the lenscapsule (cornea and iris) (not shown) from accidental contact withcutting electrode 138 thus ensuring its safe use during capsulotomy.

[0086] The apparatus and method of the invention ensure efficientthermal ablation at low power levels, e.g., ranging down to 10 mW byoverheating and evaporation. Devices built in accordance with theinvention can be used for cutting various types of materials includingbiological tissue while minimizing the damage zone and minimizing heatlosses into the material being cut as well as the surroundings and thehand piece. The voltages necessary for producing the plasma are reducedsignificantly in comparison to prior art devices. Because of such powerefficiency and low thermal damage the apparatus of invention and methodfor operating it can be adapted to numerous applications in surgery onvery sensitive organs, such as the eye. For example, the apparatus ofinvention can be used for: (a) dissection of membranes and cuttingretina in vitreoretinal surgery, (b) capsulotomy, (c) lensectomy, (d)iridectomy, (e) trabeculectomy.

[0087] A person skilled in the art will recognize that many extensionsand alternative embodiments of the invention are possible and that thefull breadth of the invention is hence defined by the scope of theappended claims and their legal equivalents.

We claim:
 1. A method for cutting a material submerged in a conductiveliquid medium, said method comprising: a) providing a cutting electrodehaving an elongate cutting portion; b) providing a return electrode; c)immersing said cutting electrode and said return electrode in saidconductive liquid medium; d) applying a voltage between said cuttingelectrode and said return electrode such that said conductive liquidmedium is heated to produce a vapor cavity around said elongate cuttingportion and to ionize a gas inside said vapor cavity to produce aplasma; e) modulating said voltage in pulses having a modulation formatselected to minimize the size of said vapor cavity, a rate of formationof said vapor cavity, and a heat diffusion into said material; and f)cutting said material with an edge of said elongate cutting portion. 2.The method of claim 1, wherein said format comprises pulses having apulse duration selected in the range substantially between 10 μs and 10ms.
 3. The method of claim 2, wherein each of said pulses comprisesminipulses having a minipulse duration selected in the range between 0.1and 10 μs and an interval between said minipulses selected in the rangebetween 0.1 and 10 μs.
 4. The method of claim 3, wherein each of saidminipulses comprises micropulses having a micropulse duration selectedin the range between 0.1 and 1 μs.
 5. The method of claim 3, whereinsaid interval is shorter than a lifetime of said vapor cavity.
 6. Themethod of claim 3, wherein said minipulse duration and a peak power areadjusted to permit spark discharges and to prevent arc discharges. 7.The method of claim 3, wherein said minipulses exhibit alternatingpositive and negative polarities.
 8. The method of claim 2, wherein thevoltage of said pulses is varied during said pulse duration, such that alow voltage is applied for electro-chemical generation of said gas and ahigh voltage is applied for generation of said plasma.
 9. The method ofclaim 1, wherein said voltage and said format are selected such that thetemperature of said elongate cutting portion and said plasma aremaintained significantly above the boiling temperature of water.
 10. Themethod of claim 9, wherein the temperature of said elongate cuttingportion is maintained between about 100 and 1,000° C.
 11. The method ofclaim 1, further comprising preventing charge transfer to said material.12. The method of claim 1, wherein said material is selected from thegroup consisting of biological tissue, cellulose and plastics.
 13. Themethod of claim 1, wherein said elongate cutting portion has an aspectratio of length to width larger than
 1. 14. The method of claim 13,wherein said aspect ratio is larger than
 5. 15. The method of claim 1,wherein said elongate cutting portion has a width between 1 and 250microns.
 16. The method of claim 15, wherein said elongate cuttingportion has a width between 10 and 100 microns.
 17. The method of claim1, wherein said elongate cutting portion is a wire with diameter between1 and 250 microns.
 18. The method of claim 17, wherein said elongatecutting portion is a wire with diameter between 10 and 100 microns. 19.An apparatus for cutting a material submerged in a conductive liquidmedium, said apparatus comprising: a) a cutting electrode having anelongate cutting portion; b) a return electrode; c) a voltage source forapplying a voltage between said cutting electrode and said returnelectrode such that when said cutting electrode and said returnelectrode are immersed in said conductive liquid medium said conductiveliquid medium is heated to produce a vapor cavity around said elongatecutting portion and to ionize a gas inside said vapor cavity to producea plasma; and d) a pulse control for modulating said voltage in pulsesof a modulation format selected to minimize the size of said vaporcavity, a rate of formation of said vapor cavity, and heat diffusioninto said material.
 20. The apparatus of claim 19, wherein said pulsecontrol comprises a peak power control and a duration control and saidformat comprises pulse power, pulse duration and pulse interval.
 21. Theapparatus of claim 20, further comprising a low impedance lineconnecting the output of said voltage source and said elongate cuttingportion for producing micropulses.
 22. The apparatus of claim 19,wherein said material is selected from the group consisting ofbiological tissues, cellulose and plastics.
 23. The apparatus of claim19, wherein said elongate cutting portion has a width between 1 μm and250 μm.
 24. The apparatus of claim 23, wherein said width ranges between10 μm and 100 μm.
 25. The apparatus of claim 19, wherein said elongatecutting portion has a circular cross section.
 26. The apparatus of claim19, wherein said elongate cutting portion has at least one bend.
 27. Theapparatus of claim 19, wherein said elongate cutting portion forms aloop.
 28. The apparatus of claim 19, wherein said cutting electrodecomprises a wire electrode.
 29. The apparatus of claim 28, furthercomprising a means for advancing and retracting said wire electrode. 30.The apparatus of claim 19, wherein said material comprises biologicaltissue and said apparatus further comprises a device for preventingcharge transfer to said biological tissue.
 31. The apparatus of claim30, wherein said device comprises an RC-circuit.
 32. The apparatus ofclaim 30, wherein said elongate cutting portion is designed forcapsulotomy and said biological tissue comprises eye tissue.
 33. Theapparatus of claim 19, wherein said elongate cutting portion has anaspect ratio of length to width larger than
 1. 34. The method of claim33, wherein said aspect ratio is larger than 5.